WINETECH Technical Yearbook 2019

Color Key Col u

to apply for Chardonnay, a cultivar that is typically found to have very high average YAN concentrations. When arranging the percentage of arginine content from least to most and proline from most to least, in most cases the cultivars line-up or are relatively close to lining up (Figure 2). Therefore, it appears that proline and arginine concentrations are, to a degree, inversely proportional to one another. Some early studies on amino acid profiles have suggested that the proline to arginine ratio can be used as an index to discriminate between cultivars. The ratio of proline to arginine as a cultivar indicator was, however, not appropriate when large variations were found in the juices surveyed. PREDICTIVE ABILITY OF THE GRAPE MUST AMINO ACID PROFILE The data was used to eva luate how accurately the amino acid composition could be used to discriminate between cultivars and predict a certain cultivar. Details on the statistical aspects of this work are detailed in a recently published MSc thesis. For the purpose of this article, however, we will present the process and the outcome in a simplified version. Step 1: Red or white? As a first step, for the discrimination between white and red grape juices, alanine, leucine, GABA and proline were the amino

-2 -1 0 1 2

Column Z-Score

Shiraz

Cabernet franc

Roussanne

Chardonnay

Merlot

Cabernet Sauvignon

Cinsaut

Viognier

Pinotage

FIGURE 1. Heat map of the average amino acid concentrations and dendrogram illustrating how these cultivars relate to one another based on these average concentrations.

Chenin blanc

Sauvignon blanc

Sémillon

Grenache blanc

ALA

ARG

ASP

GABA

GLN

GLU

GLY

HIS

HYP

ILE

LEU

LYS

MET

ORN

PHE

PRO

SER

THR

TRP

VAL

Grenache blanc, Sémillon, Sauvignon blanc and Chenin blanc appear to group together based on the lower concentrations of amino acids compared to the other cultivars included in this study. Furthermore, it is clear that Merlot is the cultivar with the highest concentration of proline and that Pinotage – and to a lesser degree, Cinsaut – generally has higher concentrations of most of the amino acids compared to the other cultivars surveyed. The close genetic relationship between Pinotage and Cinsaut,

together with the similarity in the amino acid profile, highlights the influence of the genetic make-up in determining the grape must composition. PROLINE AND ARGININE Studies profiled cultivars according to whether they were proline or arginine accumulators, with proline accumulators indicated by a ratio of >1 and arginine accumulators indicated by a ratio of <1. It was proposed that this ratio could be used as an indicator of the ratio of assimilable

nitrogen to non-assimilable nitrogen. (If you remember, arginine is a primary amino acid and therefore included in the FAN/YAN value, while proline is a secondary amino acid and does not contribute to this value.) This rule seems to hold up for most cultivars. For example, Grenache blanc, Pinotage and Cinsaut are all high-YAN yielding cultivars with a proline to arginine ratio of <1, and vice versa for cultivars, such as Merlot, Cabernet Sauvignon and Cabernet franc. On the other hand, this rule does not appear